The subject matter of the present invention relates to a new method and apparatus for performing an interactive formation evaluation of laminated reservoirs on a workstation, and more particularly, to a new method and apparatus for defining more accurately layer boundaries of a highly laminated reservoir traversed by a wellbore and for providing a more accurate quantitative petrophysics evaluation of a highly laminated reservoir.
When a well logging tool is disposed in a wellbore, it measures the properties of a formation traversed by the wellbore and generates output signals representative of these properties, the output signals being transmitted uphole to a well logging truck situated at the wellbore surface and printed or recorded in the form of output record logs on an output record medium. These logs are subsequently input into a separate workstation where they are analyzed by interpretation software for the purpose of determining the specific properties of the formation, that is, whether the formation consists of oil, water, gas, quartz, sand, etc. For example, the interpretation software responds to the receipt of the output record logs by performing a volumetric analysis on such logs. When a volumetric analysis is performed, volumes V.sub.j are determined, such volumes further determining the amount of oil, water, gas, quartz, sand, etc present in the formation at the different depths of the wellbore. One such interpretation software which performs a volumetric analysis is known as Expert Log Analysis (ELAN). Equation 3 set forth in the Detailed Description of the Preferred Embodiment below represents a classical ELAN equation of formation evaluation wherein such volumes are calculated. Although the ELAN software performs its interpretation function well when the logging tool is disposed in most wellbores, when the logging tool is disposed in a highly laminated wellbore (one where the formation traversed by the wellbore consists of a multitude of layers of different materials), the ELAN software fails to record sufficiently an interface between the end of one layer of material at a particular depth of the laminated wellbore and the beginning of another layer of material at said depth. In fact, ELAN fails to distinguish and adequately record, on the output record medium, the multitude of interfaces which exist along a laminated wellbore. In addition, the ELAN software often fails to accurately record, on the output record medium, the volumes of oil, water, etc, which exist in the laminated formation at different depths in the wellbore.
Evaluation of thinly laminated reservoirs is not a new problem in formation evaluation and interpretation. See, for example, U.S. Pat. No. 3,166,709 entitled "Method and Apparatus for providing Improved Vertical Resolution in Induction Well Logging, including Electrical Storage and Delay Means" issued in 1965, the disclosure of which is incorporated by reference into this specification. Another element brought up early in the interpretation of laminated reservoirs is the notion of Model. In an article entitled "A Contribution to Electrical Log Interpretation in Shaly Sands" by Poupon A., Loy M. E., and Tixier M., Trans. AIME 1945 Vol 201, 138, 145, A Poupon et al were the first to assume a laminated conductivity model for estimating the bulk conductivity (1/Rt) probed by Deep Induction and then trying to minimize the difference between real and computed formation conductivity. Three pieces of information were necessary to compute Rt in sand: first, the detection of the sand laminations with a high resolution log (the Microlog); second, the assumption that the shale laminations had a constant conductivity equal to the surrounding massive shale; and third, the computed bulk conductivity, obtained by convolving the conductivity model with the Induction "Geometrical Factor Map". Therefore, from the beginning, it was clear that the resolution of Rt is one of the main limitations in estimating water saturation with standard open hole logs. To succeed, any thin bed interpretation method requires: (1) at least one high resolution measurement, (2) a model and some petrophysical assumptions, (3) the means to compute or "reconstruct" the tool measurement, given the assumed model.
In the Laminated Sand Analysis (LSA) developed by D. Allen (see "Laminated Sand Analysis" D. F. Allen, SPWLA 25th Annual Logging Symposium, 1984) for shaly sand reservoirs drilled with fresh or oil base mud (see "Predicting Hydrocarbon Saturations in Thin Sandstones drilled with oil-based mud" M. Bilsland, R. Mobed, E. Cheruvier, SAID Paris, Oct. 24-25-26, 1989), Rt is obtained from a Dual Induction. Electromagnetic wave Propagation (EPT) attenuation provides the high resolution information necessary to detect the laminations but also to discriminate between them. Indeed, when the conductivity of the invading fluid is low and much smaller than the conductivity of the shale bound water, EPT attenuation is approximately a linear function of the volume of conductive water within the flushed zone. EPT attenuation is therefore used to estimate the amount of bound water in the shale at a high resolution scale. Finally, the Deep Induction measurement is reconstructed by convolving its vertical response function with the proper vertical sequence of sand and shale laminations. Porosity, which enters into Archie's bulk conductivity equation, is derived from a Density Neutron porosity measurement. An apparent deep fluid conductivity can therefore be estimated versus depth which should lie between the conductivity of a wet clean sand and a 100% shale. The interpreter can modify model parameters to adjust the discrimination between sand and shale.
An alternative method was developed by J. Suau et al (see "Interpretation of Very Thin Gas SAnds in Italy" Suau J. et al, SPWLA 25th Symposium, 1984) which made use of pattern recognition and correlations computed between EPT, Neutron, Density, and Sonic logs to evaluate tight gas sands.
Several techniques have been developed to enhance a low resolution measurement by combining it with another measurement which has a much better vertical resolution but has generally a smaller depth of investigation. The best results have been obtained when the two measurements are measuring the same physical parameters. For example, the enhanced Phasor Deep Induction (see "Induction Log Vertical Resolution Enhancement-Physics and Limitations" Barber T., Trans., 1988 SPWLA, San Antonio) achieves higher vertical resolution by processing the standard Medium Phasor Induction to enhance the standard Deep Phasor Induction. High frequency information derived from the Medium is combined with the Deep phasor. In the case of the Neutron log (see "Enhanced Resolution Processing of Compensated Neutron Logs" Galford J. E., Flaum C., Gilchrist W. A., and Duckett S. W., paper SPE 15441, 1986 SPE, New Orleans) or the Density Log (see "Enhanced Vertical Resolution Processing of Dual Detector Gamma-Gamma Density Logs" Flaum C., Galford J. E., SPWLA 1987, 28th Annual Logging Symposium), this approach, known as "alpha processing", is performed in two steps. First, the high resolution measurement is low-pass filtered to yield a spectrum which matches the low resolution one. This produces a gain and offset on the high frequency measurement. Second, the enhanced high resolution output is computed as the sum of the low resolution log and the difference between the normalized high resolution measurement and the matched version of the original low resolution log.